This invention relates generally to low emission gas turbine engines and in particular to a system and method for minimizing emissions from these engines.
Air pollution concerns worldwide have led to stricter emissions standards requiring significant reductions in gas turbine pollutant emissions for both industrial and power generation applications burning either liquid or gaseous fuel.
Among other factors, the emissions from a gas turbine engine are dependent on the peak flame temperature in the engine""s combustor. It is well known that higher peak flame temperatures result in lower carbon monoxide (CO) emissions and higher nitrous oxide (NOx) emissions while lower peak flame temperatures result in higher carbon monoxide emissions and lower nitrous oxide emissions. Depending on the type of fuel used, there is a peak flame temperature wherein the combination of carbon monoxide and nitrous oxide is minimized. This relationship is illustrated in FIG. 1 in which curves of CO and NOx emissions, in parts per million, are plotted versus peak flame temperature. From this curve an ideal peak flame temperature can be selected, represented by the dashed line, that corresponds to optimum emissions of the combination of CO and NOx. Alternatively, the desired peak flame temperature can be selected either to the left or right of dashed line for improved CO or NOx emissions at the expense of the other.
This relationship between peak flame temperature and emissions suggests that a control system can be developed for a gas turbine engine that would control on-peak flame temperature to maintain the engine at the optimum emissions point. There are, however, a number of obstacles that must be overcome to make such a control system practical. First, because flame temperatures are very high, on the order of 1900xc2x0 F., (1038xc2x0 C.), conventional thermocouples or other sensors used to measure peak temperature cannot operate in this harsh environment for very long and are prone to failure. Second, because the thermodynamics and fluid dynamics of the combustion process are very complex, it can be difficult to physically locate in the combustor where the peak flame temperature is. Furthermore, the location of the peak flame temperature may move within the combustor as the engine operating point changes. As a result, these temperature measuring devices end up being in the wrong location in the combustor or only in the correct location at one operating point of the engine. The consequence is that the engine ends up operating off of its optimum emissions point.
Accordingly, there is a need for a system and method for operating a gas turbine engine at its optimum emissions point that does not depend on flame temperature.
An object of the present invention is to provide a system and method for operating a gas turbine engine at its optimum emissions point that does not depend on flame temperature.
The present invention achieves this objective by providing a gas turbine engine comprising, in flow series arrangement, a mixer, a compressor, a first flow path of a recuperator, a combustor, a turbine and a second flow path of the recuperator, and a bypass conduit between the turbine and the mixer. A valve is disposed in the conduit and is controlled by a temperature sensor mounted to sense compressor inlet air temperature. The sensor has a set point temperature that corresponds with optimum emissions from the engine. When the sensed temperature drops below the set point temperature the valve opens and hot air from the turbine flows to the mixer where it heats the air entering the compressor.
A method for operating a gas turbine engine with preselected emissions over a range of ambient conditions is also disclosed. This method includes the steps of designing the gas turbine engine to operate at preselected emission levels at a preselected compressor inlet air temperature, sensing the compressor inlet air temperature, and when the sensed temperature is less than the preselected temperature heating the compressor inlet air until it returns to the preselected temperature and the engine returns to preselected emission levels.
These and other objects, features and advantages of the present invention, are specifically set forth in, or will become apparent from, the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings.